The Stat transcription factors play pivotal roles in controlling the expression of genes involved in immune responses, cell transformation and maintaining homeostasis. Published results from this lab demonstrate that there is a pool of Stat3 that is localized in the mitochondria (mitoStat3) where it functions to control cellular respiration both in cells and in cardiac tissue. Preliminary results with a transgenic mice that express Stat3 that is targeted heart mitochondria indicate that it the transgenic protein protects hearts from ischemia induced decreases in the activity of complex I of the electron transport chain, production of reactive oxygen species (ROS) from mitochondria and release of cytochrome C from the mitochondria. Preliminary results in this proposal also define a new function of Stat1 as a repressor of mitochondrial gene expression. In addition, Stat1 represses the transcription of nuclear RNAs that encode components of the electron transport chain. It appears that the actions of Stat1 might antagonize the effects of Stat3 in regulation of mitochondrial homeostasis as well as their well knows opposing actions on cell growth and inflammation. Experiments are proposed to define the contribution of mitoStat3 to the cardio-protective effects of this transcription factor in acute and chronic models of heart injury. Since the mechanisms by which Stat1 represses mitochondrial transcription leading to decreased mitochondria function appear to oppose the effects of mitoStat3, we will examine if mitochondrial targeted Stat3 transgenes show enhanced protection in a Stat1-/- background. Completion of these studies will elucidate a mitochondria-nuclear signaling network that is regulated by Stat3's location in the both the nucleus and mitochondria.